1. Field of the Invention
This invention relates to a blowout mode switching mechanism for an automotive air conditioning system.
2. Description of the Related Art
Japanese Unexamined Patent Publication No. 2004-155263 proposes a conventional automotive air conditioning system in which a defroster opening 20, a center face opening 21a and a foot opening 22 are opened/closed by two rotary doors 25, 26 as shown in FIGS. 10 to 12.
Specifically, the foot opening 22 and a defroster/face communication path 27a are opened/closed by the first rotary door 25, and the defroster opening 20 and the center face opening 21a are arranged downstream of the communication path 27a and are opened/closed by the second rotary door 26.
The rotary doors 25, 26 have door surfaces 25c, 26c rotatable integrally with each of the rotary shafts 25a, 25b, 26a, 26b. The door surfaces 25c, 26c are positioned a predetermined distance radially away from the center of each of the rotary shafts.
FIGS. 10, 11 show the foot mode in the prior art. FIG. 10 shows the maximum heating state in which a cool air path 16 is closed up by an air mix door 14, and FIG. 11 an intermediate temperature control state in which the air mix door 14 is operated at the intermediate opening degree position.
In foot mode operation, the first rotary door 25 is operated to a position rotated clockwise by a small angle θ from the closed-up position of the defroster/face communication path 27a. As a result, a gap X is formed between the first rotary door 25 and a case-side seal surface 11g. Therefore, the first rotary door 25 opens the communication path 27a by a small amount at the gap X while at the same time opening the foot opening 22 almost fully.
In the process, the second rotary door 26 fully opens the defroster opening 20 and closes the center face opening 21a. Most of the air-conditioning air adjusted in temperature by the air mix door 14 flows toward the foot opening 22 and is blown out, toward the feet of the occupants, from the foot air outlets 24a, 24b. 
Part of the air-conditioning air flows toward the defroster opening 20 through the gap X portion of the communication path 27a and exhibits the function of defrosting the window glass of the automotive vehicle. FIG. 12 shows the intermediate temperature control state of the foot/defroster mode in which the defroster/face communication path 27a and the foot opening 22 are both half opened by the first rotary door 25.
In the case where the first rotary door 25 changes the stop position (rotational angle θ) thereof under the air pressure in foot mode, the size of the gap X also changes. Therefore, the proportion in the blowout air amount between the defroster opening 20 and the foot opening 22 undesirably undergoes a change. As a result, it becomes difficult to maintain an appropriate preset proportion between the defroster blowout air amount and the foot blowout air amount.
Also, in the intermediate temperature control state in the foot mode shown in FIG. 11 or the foot/defroster mode shown in FIG. 12, the opening of the defroster/face communication path 27a is in proximity to the cool air path 16 and the foot opening 22 is in proximity to the outlet of a warm air path 18 downstream of a heater core 15. Therefore, cool air is liable to flow from the cool air path 16 toward the defroster opening 20, while the warm air in the warm air path 18 is liable to flow toward the foot opening 22.
As a result, the temperature difference (vertical blowout temperature difference) between the blowout air temperature of the defroster opening 20 and the blowout air temperature of the foot opening 22 becomes excessive, so that the defrosting performance of the defroster opening 20 is deteriorated by the excessive reduction in the blowout air temperature. Also, the excessive rise of the blowout air temperature at the foot opening 22, on the other hand, inconveniently deteriorates the comfort of the occupants.